In recent years, the production of coatings through thin and uniform application of various coating liquids has been strongly demanded to form coatings over plastics substrates for optical filters, glass substrates for liquid crystal displays, and glass substrates for color filters, etc. and to form photoresists or protective layers on printed circuit boards or wafers, etc. in an integrated circuit or semiconductor manufacturing process. This requires the industrial-scale production of coatings on small-size substrates, in many cases less than 1 meter long in the coating direction, and necessitates the adoption of a sheet coating method which involves the feeding of substrates to the coater, one by one, application of coating liquids, and transfer of the coated substrates to the next process such as drying.
The methods which have been used conventionally and widely for such coating include the use of a spin coater, bar coater and roll coater.
Of these, the spin coater method which is widely used to form photoresist over a semiconductor wafer can apply coatings on a spinning substrate to be coated by dropping a droplet of coating liquid at the center of the substrate and spreading it over the surface by means of a centrifugal force. This method can produce uniform coatings over the entire surface of a substrate to be coated with a high thickness accuracy by choosing coating liquids suitable for this method. With the method, however, only several to ten percent of the coating liquid dropped on the surface of the substrate can be utilized for the actual formation of a coating, and the remainder, more than 90%, of that is removed from the surface and thrown away. Thus, a very large amount of coating liquid is required to obtain a film with a predetermined thickness, making the method uneconomical. In some cases, moreover, the coating liquid is deposited on an edge or the bottom surface of the substrate, or waste coating liquid scattered within the equipment gels or solidifies, which reduces stability and cleanliness, leading to degradation in the quality of the coated product.
The roll coater method involves the transfer of a coating liquid onto the surface of a substrate to be coated via a rubber roll, and is capable of applying a coating on a long material or on a continuous material wound onto a reel. However, since the coating liquid is supplied from a pan to an application roll and then to the substrate, exposure to the air becomes prolonged, which gives rise to vulnerability to degradation due to moisture absorption and oxidation, as well as the intrusion of foreign matter. As a result, degradation in the quality of the coated product tends to occur.
The bar coater method involves the application of a coating liquid onto a substrate to be coated using a bar made of a rod on which thin wire is wound. The problem with this method is that line marks are easily formed on the coating due to the contact between the wire wound on the rod and the coated substrate.
The die coater method, on the other hand, has been used conventionally and widely in areas where the production of thick coatings or continuous application of high-viscosity coating liquids is required. In case that a coating is formed on a substrate to be coated by using a die coater, the coating liquid is supplied through a slot of the die of the die coater to produce a pool of the coating liquid, called a coating liquid bead, between the die and the substrate which is moving relatively to the die running while maintaining a constant gap between them, and the coating liquid is pulled out as the substrate runs to form a coating, as has been disclosed, for example, in U.S. Pat. No. 3,526,535. Continuous production of a coating is possible by supplying the same amount of coating liquid as that consumed in the coating formation.
Thus, a coating produced with a die coater can achieve a uniform thickness with a considerable degree of accuracy. There is hardly any waste of coating liquid, and as the coating liquid supply path to the slot outlets is enclosed, the degradation of the coating liquid and intrusion of foreign matter can be prevented, thus enabling the method to enhance the quality of the resultant coating. This method also makes it possible to provide a rectangular-shaped coating at any desired position of a substrate to be coated.
In light of these problems associated with the spin coater, bar coater or roll coater method, a proposal to use the die coater method for the manufacture of color filters has been made recently in Japanese Patent Publication Laid-Open (Kokai) Nos. 5-11105 (1993) and 5-142407 (1993).
However, these die coaters lack a substantial history in their application to sheet substrates and are not sufficiently high in the levels of coating position accuracy, film thickness accuracy, reproducibility, stability, etc., which are essential for the continuous mass production of high quality coated products.
There seem to be four major technical reasons for this.
Firstly, adequate consideration has not been given to the formation and disappearance of a coating liquid bead, despite their importance for stable coating operations.
Namely, when a die coater is used to form a coating on substrate fed in a sheet-form, the application of the coating liquid inevitably becomes intermittent, so that disturbance of a coating liquid bead or disappearance of a coating liquid bead occurs at the start-of-coating line and/or the end-of-coating line on the substrate, regardless of whether the coating liquid is discharged continuously or intermittently. This makes it difficult to maintain a stable and suitable coating liquid bead over the entire coating area, and a uniform coating cannot be achieved until the bead reaches a stable state. If the stabilization of the bead requires a long time, it will lead to an increase in the area where the coating thickness is uneven, and the portion of the substrate which can be used effectively becomes extremely small. Regarding the formation and disappearance of a coating liquid bead, a method of producing a connecting bead, i.e. coating liquid bead, by generating pulses in supplying coating liquid has been disclosed in U.S. Pat. No. 4,938,994. However, by this method, the start-of-coating line cannot be accurately fixed since the substrate is moving while a coating liquid bead is formed and stabilized, and the length of the coated portion of the substrate before a coating liquid bead has been stably formed increases, thereby decreasing the portion of the substrate over which the required film thickness is obtained uniformly.
Secondly, no consideration is given to the relative positions of the substrate and the slot of the die. Where shifts occur to their relative positions or their reproducibility is poor, the position of the coated area may also shift, possibly with large fluctuations well beyond the allowable range. This is particularly crucial when a rectangular coating is to be formed on an inside portion of the surface of the substrate.
Thirdly, adequate consideration is not given to achieving a uniform clearance, i.e. the distance between the substrate and the exit face of the slot of the die, which has a major impact on the maintenance of a coating liquid bead.
Namely, when producing a coating with a uniform thickness on a substrate to be coated by using a die coater, the clearance must be kept constant over the entire width of the die of the die coater. The conventional way of keeping the distance from the substrate constant over the entire width of the die of the die coater is to measure the parallelism between the die and the substrate with a gauge etc. while the die is mounted on its support, and, if the parallelism between them is not satisfactory, manual adjustments to the condition of the die mounted on the support are made. Dies need to be washed regularly, since their continuous use gradually renders their interior dirty. However, if the adjustment work necessary after the mounting of the washed die onto the die coater is undertaken manually, it becomes cumbersome and requires a considerable amount of time to complete, which reduces the productivity. With manual adjustment, the accuracy of clearance depends on the workmanship of individual workers, making it impossible to always achieve a required accuracy with high reproducibility. In particular, when a thin coating is to be formed, a minute deviation in parallelism created through the adjustment process results in a large fluctuation in the thickness of the coating produced, greatly reducing the quality of the coating.
Moreover, the substrate itself fluctuates in thickness, and in addition, the vertical movement of the table carrying the substrate causes fluctuations in the clearance as the substrate travels. Depending on the severity, this can constitute an obstruction to improving the accuracy of coating thickness.
Usually, the linear slider which guides the table is provided by a linear motion guide. A linear motion guide here refers to a mechanism in which numerous balls are provided in such a way that not only can each of them rotate on its axis but they can also circulate along a predetermined path (hereinafter referred to as a revolution), so that the table can be moved smoothly as a result of the rotation and revolution of these balls.
However, when a table with a linear slider composed of a linear motion guide is used, the vertical movement of the table cannot be reduced to a low level because it undergoes considerable pitching and yawing. As a result, fluctuations in the clearance become large, making it impossible to control the coating thickness with high accuracy, i.e. to apply a uniform coating over the entire surface of the substrate.
A likely solution to this is the use of roller bearings in place of a linear motion guide to improve the traveling accuracy of the table, i.e. to reduce its vertical movement. However, as the traveling speed of the table increases up to a certain high-speed region, slipping starts to occur between the table support and roller bearings, which causes eventually the table supports to run off from the rollers, and a problem in that it is incapable of prolonged use under high-speed conditions.
Fourthly, there have been problems associated with the drying and heat curing of the coating liquid in the manufacturing of coated sheet products such as color filters, as described below.
Conventional methods of manufacturing coated sheet products such as color filters usually include drying and heat curing, by the oven method in which a coating liquid is applied over a glass substrate using a spin coater and then heated with the coated glass substrate held in a heated atmosphere, and by the hot plate method in which the coated glass substrate is placed on a hot plate. Coating by means of a spin coater takes some 60 seconds, and in addition, a considerable amount of the solvent in the coating liquid evaporates to accelerate the drying while the excess coating liquid is dissipated. This increases the concentration and viscosity of the coating liquid, resulting in a low fluidity at the end of the coating process. Therefore, the use of the oven or hot plate method to dry and heat-cure coatings rarely results in the spoiling of the coated surface due to external disturbances such as changes in the evaporation pattern, uneven temperature distribution and convection.
However, if a die coater and a spin coater are used to apply the same coating liquid on a glass substrate, the die coater is much shorter in the coating time compared with the spin coater, and in the absence of any particular factors which contribute to accelerated evaporation, the solvent does not evaporate much before the end of the coating process, so that the concentration, viscosity and liquidity of the coating liquid remain almost unchanged. Therefore, the use of the same drying and heat-curing method as in the case of a spin coater has so far resulted in coating defects. Namely, when the coating liquid is heat-cured using the hot plate method, marks of several pins used to support the glass substrate, marks of the arm used to convey the substrate and marks of the hot plate notches provided for the conveyance tend to be left undesirably on the coating. This problem occurs as the pins, arm and notches come into contact with the glass substrate, and this causes an uneven temperature distribution due to localized increases or decreases in the temperature of the affected parts of the glass substrate, resulting in a variation in the evaporation speed of the coating liquid solvent over the substrate surface. With the oven method, too, surface turbulence marks and other defects due to convection sometimes occur, if the heating temperature is raised too high in an attempt to increase the drying speed. Also, both methods may cause surface defects such as glossy spots, as the history of the evaporation process of the solvent remains on the surface of the coating.
Moreover, there is no known method suitable for manufacturing a coated product which comprises a rectangular coating formed on an inner portion of a surface of a sheet substrate. The simple utilization of a conventional method is fraught with problems such as surface imperfections, and in severe cases, the edge of such a rectangular coating on a substrate cannot be kept straight as a result of the coating liquid flowing out from a part of the edge of the rectangular region.